j < Cytolytic T cell (CTL) and Th1 responses are important effector mechanisms in anti-tumor immunity. Little is known about role of B cells in regulating anti-tumor responses. IgM"'" B cell-deficient mice (BCDM) exhibit enhanced resistance to primary tumors compared to immunocompetent mice (ICM). Two of three syngeneic tumors (EL4 and MC38) regress spontaneously in BCDM while growth of a third (B16) was markedly slowed. Enhanced anti-tumor resistance of BCDM was associated with increased T cell infiltration, enhanced Th1 cytokine response and increased CTL activity. Reconstitution of BCDM with wild type but not OX40L"7"B cells results in decreased tumor resistance. The absence of B cells may result in enhanced Th1 and CTL responses to tumors. The overall goal of this proposal is to understand mechanisms leading to an enhanced anti-tumor response in the absence of B cells, and determine whether the proactive depletion of B cells in normal hosts will replicate conditions leading to an enhanced response. In Aim /we will study the anti-tumor T cell responses seen in ICM and BCDM and delineate the T-cell responses involved in augmentation of tumor resistance by depletion of lymphoid subsets and/or adoptive transfer experiments. The potential role of CD4+ CD25+ T regulatory cells will also be investigated. In Aim II, we will examine the immunoregulatory mechanisms underlying inhibition of anti-tumor immunity by B cells. We will study the effects of reconstitution with wild type and OX40L"'" B cells on expansion and differentiation of tumor specific CD4+ and CD8+ T cells and anti-tumor response in BCDM. We will identify B cell effector subsets that may play a role in inhibiting anti-tumor responses. We will determine whether additional candidate B cell-derived factors or receptors such CD40, IL-10, and TGF-3 may play a role in inhibiting anti- tumor immunity. We will examine whether antigen presenting cells (dendritic cells) in BCDM have altered properties that may lead to the enhanced tumor immunity. In Aim III, we will develop murine models for pro- active B cell depletion and study the effect of B cell depletion on anti-tumor immune responses, alone or in combination with other immune intervention strategies such as cytokine delivery, gene transfer, and/or adoptive T cell transfer. Effects of B cell depletion using Rituximab and additional agents will be modeled in a human CD20-BAC transgenic mouse which mimics normal patterns of CD20 expression in B cells. We will test direct B cell depletion or depletion following transplant of huCD20-BAC transgenic bone marrow into irradiated host mice. Since B cells can be efficiently and safely depleted in man using monoclonal antibodies such asRituximab, strategies developed in Aim III canreadily be applied to design of human Phase l/ll trials.